Advanced technology engines are moving towards the elimination of oil lubricated bearings, seals, gears and sumps. Oil lubricated engines have disadvantages in regard to shaft speed and temperature, as well as reliability and maintainability. There has been progress in the use of magnetic or foil air bearings for shaft support which eliminates the need for conventional bearings, sumps, seals, and the like. There is still however, a need to couple concentric sections of engine shafting to drive externally mounted accessories, or to provide modularity to the engine. This capability is normally met with oil or grease lubricated spline couplings. These couplings, however, require lubrication to prevent excessive wear of the metallic spline parts, and are temperature limited. A coupling is needed that can provide for misalignment between the shafts, transmit the maximum torque required of the shaft, operate at elevated temperatures ( greater than 400 F.) and allow the shafts to be separated on disassembly, while requiring no lubrication during operation.
Oil-less engine technology increases power density, improves specific fuel consumption (SFC), and reduces cost of ownership for gas turbine engines. The elimination of lubrication, seals, oil collers, pumps, scavenge systems and the like, utilizing foil air and/or magnetic bearings can produce significant cost, weight and sfc improvement. Oil-less bearings can be designed for higher temperature, higher Dn environments than were achievable with oil lubrication, where Dn=bearing inner diameter (mm)xc3x97rotational speed (rpm). Shaft couplings transmit torque between shaft sections, allow easy removal of accessories, accommodate minor shaft misalignment, and provide for a modular engine design. In addition to running without oil or grease, a new generation of shaft couplings with increased temperature capability is needed, for example, to allow for xe2x80x9chot end drivesxe2x80x9d, where shaft power is extracted from the aft, hot end of the engine.
U.S. Pat. No. 3,620,043 for Spline-Type Pivots, Universal Joints and Flexible Couplers, discloses a typical spline joint. The device consists of a cylindrical insert made from self-lubricating polymeric material such as Amoco Torlon or Dupont Vespel. The part is made with an involute male spline on its OD, and a circular spline on the ID. The insert is installed with an interference fit into the mating female metallic spline. The male metallic spline engages the female circular spline in the insert during installation of the accessory, or shaft section. This type of insert provides the following benefits over a conventional oil or grease lubricated metallic spline pair: 1) It can be used as a replacement for worn involute spline couplings without modification or replacement of the accessory drive shaft; 2) it can be used dry, in applications where oil or grease spline lubrication is not used; and 3) the insert is sacrificial and is easily replaced when worn out, without any rework needed on the metallic components.
A significant limitation of this spline coupling, however, is the temperature capabilities of the adapter material. Torlon and Vespel are capable of maximum usage temperatures in the range of 450xc2x0 F. This must include ambient temperature, and frictional heating of the spline adapter during operation.
U.S. Pat. Nos. 5,860,890; 6,042,500 and 6,006,885 each discloses the use of carbon-carbon composite material to transmit torque. Each of these patents disclose such use in regard to an automotive automatic transmission friction clutch disk. None of these examples of prior art relating to carbon-carbon composite material pertains to or suggests the use of such material as a high temperature, high strength, wear-resistant spline-type coupling insert, to provide an oil-less, self-lubricating tubular insert spline device for gas turbine engines.
A cylindrical coupler or coupling for joining the facing ends of coaxial shafts is made from carbon-carbon composite with a female spline in its bore and a male spline on the outer surface. Variations of the torque carrying geometry include involute splines, circular splines, square drive, hex drives, serrations, or double-D drives. By fabricating this coupler from carbon-carbon (or carbon-graphite) composite, metallic shaft mating splines are provided with a coupler that has exceptionally high temperature capabilities ( greater than 2500 F.), high strength, and can be impregnated with graphite or other dry lubricants to reduce frictional heat generation and reduce spline wear. The coupler should provide long life, and is easily replaced.
A preferred embodiment of a method for fabricating the inventive coupler comprises the principal steps of forming a shaped tubular blank of carbon-carbon composite material and machining the blank to provide mating surfaces to receive the facing end of co-axial shafts. An additional step may then include impregnating the machined blank with dry lubricant. The dry lubricant may include graphite, molybdenum disulphide and combinations thereof.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and claims.